In nozzle-type centrifugal separators, known as disk-nozzle centrifuges, the separated underflow is discharged through nozzle means arranged at the outer periphery of the separating chamber in the centrifugal bowl. The centrifuge effects a two-fraction separation of a feed slurry into a heavy nozzle discharge slurry or the so-called underflow fraction or concentrate delivered by the nozzles, and a light fraction or separated liquid delivered from the overflow bowl at the top end of a machine. It is the liquid overflow which is the desired end product and must have its solids content carefully regulated. Part of the underflow fraction is recycled to the separating chamber at a controllable rate, by introduction through the lower end of the rotor bowl. In use of such separators it is often necessary to control the solids content of the discharging underflow by such recycling to the separating chamber. The common use of underflow recycling is in cases where the feed to the centrifuge has a low content of solids, and the desired result is a high concentration of solids in the underflow slurry. There is a need for precise control in these cases when the feed to the centrifuge is altered or when the underflow contains too high a concentration of solids so as to cause plugging of the discharge nozzles.
One solution to the problem was put forth by the underflow concentration control for nozzle centrifuges disclosed in U.S. Pat. No. 4,505,697, issued to Lee et al in 1985. The prior system utilizes means for regulating the quantity of recycle in response to an increase in the viscosity of the underflow. More particularly, the underflow containing a given concentration of solids will exhibit a certain viscosity as it flows through the duct means, and with constant viscosity the underflow will remain at a fixed rate. As the solids content increases, the resulting increased viscosity of the underflow causes it to flow at a reduced rate through the duct means, thus reducing the amount of underflow recycled through the centrifuge and counteracting the increase in viscosity. In this way, the prior art device holds the concentration of solids in the underflow substantially constant.
This prior art remedy addressed a constant underflow concentration, whereas the underflow control apparatus embodying the present invention achieves an optimal control for a disk nozzle centrifuge. This optimal underflow control mechanism adjusts the volume of the recycle flow and thereby the underflow for different feed and underflow concentration situations.
Another prior art system for controlling the underflow from nozzle centrifuges is disclosed in U.S. Pat. No. 4,162,760, issued to Hill in 1979. The manual system uses an adjustable head sump for recycling with an adjustable toroidal ring-type valve. The device is not viscosity or flow sensitive nor does it attempt to create or maintain an optimal solids concentration in the overflow due to alteration in feed volumes and underflow solids concentrations.
Real processes have feed concentrations that constantly vary and, consequently, the optimal underflow operating point will vary with each change. Process control is needed to maintain the optimal underflow concentration. The prior art devices, including the two previously identified, are not able to control the centrifuge in the optimal sense. The two general types of disk-nozzle process controls identified are the manual valve (non-automatic) and constant underflow control. These two concepts are depicted in FIG. 1. In the manual control situation, the recycle stream is set to a certain rate by manually setting a hand valve. By fixing the recycle rate, any feed solids change will change the underflow solids concentration. This is depicted by line N-N' in FIG. 1. If one initially sets the valve at U2 which is the optimal point for feed F2, then any change in the feed solids will draw the operation away from optimality. The logical consequence of no control is that the overflow will at some point in the operation, exceed the specification limit for solids. Operators, as a result thereof, have chosen to operate (with no control) at U.sub.B, as shown in FIG. 1. This point is not optimal for feed F2 but it creates a buffer so that the overflow will always meet the desired product specification.
Constant underflow control is shown as line C-C' in FIG. 1. The control scheme is better than no control at all but is still far short of optimal. The method can be enhanced if combined with the aforementioned buffer concept. As discussed above, Lee et al (4,505,697) is an example of such a device. The device does not reach its desired constant underflow control target. Its poor control characteristics are shown in FIG. 2 as "Viscosity Induced Underflow Control."
It is an object of the present invention to obtain a constant overflow solids concentration through use of an optimal underflow control system for disk nozzle centrifuges which efficiently alters underflow solids concentration for different feed conditions.